1. Field of the Invention
The present invention relates to a structure for mounting a semiconductor device wherein a surface mount semiconductor device provided with bumps (protruded electrodes) is electrically and mechanically connected with a circuit board provided with circuit electrodes, and a method of mounting the same. Further, the invention is concerned with a semiconductor device suitable for use in obtaining the structure for mounting, and a method of fabricating the semiconductor device.
2. Description of the Related Art
Surface mount semiconductor devices have come to be in widespread use as a semiconductor device making up an integrated circuit (IC), a large scale integrated circuit (LSI), and so forth.
Among the surface mount semiconductor devices, there is one provided with a multitude of bumps placed in lines on the surface thereof for electrical and mechanical connection with circuit electrodes of a circuit board when mounting the same on the circuit board provided with the circuit electrodes.
FIG. 35 is a schematic view showing the construction in section of a semiconductor device provided with bumps formed in a straight-wall shape by way of example.
With the semiconductor device 10, a multitude of electrode pads 14 for connecting integrated circuits (not shown in the figure) with an external circuit are provided on the surface (in FIG. 35, the upper face) of a semiconductor chip 11, with the integrated circuits formed therein, in such a way as to run along side edges of the semiconductor chip 11 in the direction orthogonal to the plane of the figure. In FIG. 35, there is shown only one of a plurality of the electrode pads 14 placed in respective lines running along the side edge on both sides of the semiconductor chip 11.
An insulation film 16 having an opening formed over the respective electrode pads 14 in such a way as to cover the peripheral region of the respective electrode pads 14 while exposing the inside of the respective peripheral regions is formed across the surface of the semiconductor chip 11. A lower electrode 19 is provided so as to be in intimate contact with the peripheral region of the respective openings of the insulation film 16 and an exposed part of the respective electrode pads 14. Further, on top of the each lower electrode 19, a bump 22 formed in a straight-wall shape is provided.
There are also bumps formed in the shape of a mushroom wherein the top part of the respective bumps is larger in width than the base thereof. However, the bumps formed in a straight-wall shape are more suitable for reducing lateral spread thereof along a semiconductor chip, and to that extent, placement density of the bumps can be increased, so that a pitch at which connection thereof with an external circuit is made can be rendered minuscule.
FIG. 36 shows a conventional mounting structure wherein a semiconductor device provided with such bumps is mounted on a circuit board provided with circuit electrodes.
In FIG. 36, the semiconductor device 10 in a posture inverted from that shown in FIG. 35 is disposed on a circuit board 26 such that the bumps 22 are positioned on the underside of the semiconductor device 10, thereby bonding the semiconductor device 10 with the circuit board 26 by use of an anisotropic conductive adhesive 44.
The anisotropic conductive adhesive 44 is composed of a multitude of electrically conductive particles 42 dispersed in an insulating adhesive 46, and the respective bumps 22 are electrically connected with the respective circuit electrodes 28 on the circuit board 26 by the agency of the electrically conductive particles 42 while the semiconductor device 10 is mechanically connected with the circuit board 26 by agency of the insulating adhesive 46.
Herein, a method of fabricating the conventional semiconductor device shown in FIG. 35 is briefly described with reference to FIGS. 33 and 34, and a conventional method of mounting the same is described with reference to FIG. 36.
First, as shown in FIG. 33, across the surface of a semiconductor substrate 12 provided with integrated circuits (not shown) for a plurality of semiconductor chips and a plurality of electrode pads for connecting the integrated circuits with an external circuit, the insulation film 16 is formed, and an opening 16a is formed over the respective electrode pads 14 by photo etching method such that most of respective electrode pads 14 are exposed, leaving only the peripheral region thereof covered.
FIGS. 33 and 34 are enlarged sectional views showing only a portion (a region slightly larger than that corresponding to a unit of a semiconductor device) of the semiconductor substrate 12, respectively.
Subsequently, an aluminum film, a chromium film, and a copper film are sequentially deposited in that order by the sputtering method on the entire surface of the insulation film 16 and a portion of the respective electrode pads 14 exposed inside the respective openings 16a, on the semiconductor substrate 12, thereby forming a common electrode film 18 made up of layered films of aluminum-chromium-copper.
Thereafter, a photosensitive resin film 20 shown in FIG. 34 is formed to a thickness of 20 to 25 xcexcm on the entire surface of the common electrode film 18 by the spin coater method, and an opening 20a of substantially the same size as the respective electrode pads 14 is formed on portions of the photosensitive resin film 20, over the respective electrode pads 14, by photolithography method.
Subsequently, the semiconductor substrate 12 is immersed in a gold plating bath composed of sodium gold sulfite, and by means of electroplating using the common electrode film 18 as a plating electrode, the bump 22 shown in FIG. 34 is formed to a thickness of 15 xcexcm to 20 xcexcm on the common electrode film 18 inside the respective openings 20a of the photosensitive resin film 20.
Thereafter, the photosensitive resin film 20 is removed, the common electrode film 18 is etched using the bumps 22 as etching masks, and most of the common electrode film 18, other than portions thereof underneath the bumps 22, is removed, thereby forming lower electrodes 19 shown in FIG. 35. Thus, a structure wherein the bump 22 is individually installed over the respective electrode pads 14 with the respective lower electrodes 19 interposed therebetween.
Subsequently, a dicing step is applied to the semiconductor chips 12 whereby the semiconductor chips 12 are cut into the individual semiconductor chips 11, thereby fabricating the semiconductor device 10 as shown in FIG. 35.
In mounting the semiconductor device 10 on the circuit board, the anisotropic conductive adhesive 44 composed of the electrically conductive particles 42 dispersed in the insulating adhesive 46 made of epoxy resin is interposed between the semiconductor device 10 having a plurality of the bumps 22 and the circuit board 26 having a plurality of the circuit electrodes 28 disposed opposite to the semiconductor device 10, whereupon pressure and heat are applied between the bumps 22 and the circuit electrodes 28.
As a result, the electrically conductive particles 42 are sandwiched between the respective bumps 22 and the respective circuit electrodes 28, thereby effecting electrical connection therebetween. At the same time, the semiconductor device 10 is mechanically connected with the circuit board 26, and connections between the respective bumps 22 and the respective circuit electrodes 28 are sealed by the agency of the insulating adhesive 46.
However, with such a conventional structure for mounting a semiconductor device, and a method of mounting the same as described above, ten or more pieces of the electrically conductive particles 42 need to be maintained between the respective bumps 22 and the respective circuit electrodes 28 on the circuit board 26 in order to ensure a sufficiently small connection resistance value, for example, 0.1xcexa9 or less at the outset, and high reliability in connection therebetween.
With the electrically conductive particles 42 mixed in the insulating adhesive 46 of the anisotropic conductive adhesive 44, a bi-layered film made of nickel and gold is formed on the surface of plastic beads about 5 xcexcm in diameter.
The electrically conductive particles 42 have a connection resistance value of about 1xcexa9 per piece, and by maintaining ten pieces of the electrically conductive particles 42 between the respective bumps 22 and the respective circuit electrodes 28, it is possible to set a connection resistance value at the outset to about 0.1xcexa9, and to maintain a connection resistance value at 1xcexa9 or lower after a reliability test at high temperature and high humidity is conducted in an atmosphere at a temperature of 85xc2x0 C. and a humidity of 85% for 1000 hours.
The reason for an increase in the connection resistance value occurring after the reliability test at high temperature and high humidity is that adhesive strength of the insulating adhesive 46 is weakened due to degradation of epoxy resin based adhesive composing the insulating adhesive 46 interposed between the bumps 22 and the circuit electrodes 28, and a distance between the bumps 22 and the circuit electrodes 28 is slightly increased, thereby decreasing a connection area of the respective electrically conductive particles 42.
In order to secure ten or more pieces of the electrically conductive particles 42 between the bumps 22 and the circuit electrodes 28, the top face of the respective bumps 22 needs to have an area of 3000 xcexcm2 or more. Further, in order to ensure insulation between the bumps 22 adjacent to each other, a spacing three or more times (15 xcexcm) a diameter (5 xcexcm) of the respective electrically conductive particles 42 needs to be provided therebetween.
Accordingly, with the conventional structure for mounting the semiconductor device, and the method of mounting the same, it has been extremely difficult to connect the respective circuit electrodes 28 with the respective bumps 22 at a low connection resistance value, and to ensure reliable insulation between the bumps 22 adjacent to each other, thereby effecting connection between the respective circuit electrodes 28 and the respective bumps 22 at a minuscule pitch.
It is therefore an object of the invention to solve such problems as described above, so that, in surface mounting of a semiconductor device on a circuit board, connection resistance between bumps and circuit electrodes is rendered sufficiently small, and sufficient insulation between the bumps adjacent to each other is ensured, thereby enabling connection at high density to be effected at a minuscule pitch between the bumps.
The invention has been developed to solve the problem described above, and it is therefore an object of the invention to provide a structure for mounting a semiconductor device, a method of mounting the same, a semiconductor device suitable for use in obtaining the structure for mounting, and a method of fabricating the semiconductor device as described hereinafter.
First, the structure for mounting the semiconductor device according to the invention is a structure for mounting a semiconductor device provided with a plurality of electrode pads and a plurality of bumps individually connected with the respective electrode pads, formed on one face of a semiconductor chip thereof, on a circuit board provided with a plurality of circuit electrodes on one face thereof individually connected with the plurality of the bumps.
Further, the structure for mounting the semiconductor device is characterized in that an opposed face of the respective bumps of the semiconductor device is directly bonded with an opposed face of the respective circuit electrodes of the circuit board, diffusion bonding occurring at a bonding face therebetween.
The respective bumps are preferably formed of gold, and in a straight-wall shape, having a face thereof bonding with the respective circuit electrodes substantially planar (in a bonded state).
The method of mounting the semiconductor device according to the invention is a method of implementing the structure for mounting the semiconductor device described above, comprising:
a step of disposing the above-described semiconductor device provided with the plurality of bumps on the circuit board provided with the plurality of circuit electrodes such that the respective bumps are aligned with the respective circuit electrodes so as to be opposed to each other;
a step of bringing an opposed face of the respective bumps of the semiconductor device in direct contact with an opposed face of the respective circuit electrodes of the circuit board; and
a step of causing diffusion bonding at a contact face between the respective bumps and the respective circuit electrodes by applying a load thereto while applying ultrasonic vibration or heat.
The above-described method of mounting the semiconductor device preferably further comprises a step of lowering hardness of the respective bumps of the semiconductor device by applying heat thereto prior to the step of disposing the semiconductor device on the circuit board.
In such a case, the semiconductor device provided with the respective bumps formed of gold is preferably employed, and in the step of lowering the hardness of the respective bumps, the application of heat is preferably performed at a temperature in a range of 250xc2x0 C. to 350xc2x0 C. in a furnace into which nitrogen gas is introduced so as to render Vickers hardness Hv of the respective bumps to be in a range of 40 to 60.
Further, in the step of causing diffusion bonding at the contact face between the respective bumps and the respective circuit electrodes, heat is preferably applied thereto while applying ultrasonic vibration and a load.
In such a case, the ultrasonic vibration is preferably applied at a frequency in a range of 20 kHz to 30 kHz with output thereof in a range of 50 mW to 200 mW for each of the bumps, the load applied is preferably in the order of 30 g to 100 g for each of the bumps, and the heat is preferably applied at a temperature in a range of 150xc2x0 C. to 200xc2x0 C.
Further, the ultrasonic vibration and the load are preferably applied from a face of the semiconductor device on a side opposite from a face thereof on which the bumps are provided, and the heat is preferably applied from a face of the circuit board on a side opposite from a face thereof on which the circuit electrodes are provided.
Further, the method of mounting the semiconductor device described above preferably further comprises a step of injecting a sealing resin composed of epoxy resin in a gap between the semiconductor device and the circuit board, and applying baking treatment thereto, thereby curing the sealing resin, after the step of causing diffusion bonding at the contact face between the respective bumps and the respective circuit electrodes.
The semiconductor device according to the invention comprises: a semiconductor chip provided with integrated circuits, and a plurality of electrode pads for connecting the integrated circuits with an external circuit; an insulation film covering a face of the semiconductor chip, on which the plurality of the electrode pads are provided, and having an opening over the respective electrode pads; a mound-like (pillow-like) member provided on the central part of the respective electrode pads inside the respective openings of the insulation film; a plurality of lower electrodes each provided over the peripheral region of the respective openings of the insulation film and the respective mound-like members in such a way as to be individually and electrically conductive with the respective electrode pads through the respective openings of the insulation film; and a plurality of bumps each provided on the respective lower electrodes and formed such that the central part of the top face thereof is raised higher than the peripheral region thereof to the extent of a thickness of the respective mound-like members.
The mound-like members may be formed of a photosensitive resin, the same material as the constituent material of the insulation film, an electrically conductive material, a metal film formed by an electroless plating process, or so forth.
Further, the mound-like member may be provided on respective lower electrodes formed on top of the respective electrode pads.
The method of fabricating a semiconductor device according to the invention comprises the following steps from A to J:
A. a step of preparing a semiconductor substrate provided with integrated circuits for a plurality of semiconductor chips and a plurality of electrode pads for connecting the integrated circuits with an external circuit, and forming an insulation film having an opening over the respective electrode pads, on the surface of the semiconductor substrate;
B. a step of applying a first photosensitive resin to the entire surface of the semiconductor substrate with the insulation film formed thereon to a thickness thicker than the insulation film;
C. a step of patterning the first photosensitive resin by use of photolithography treatment such that portions thereof only on top of the central part of the respective electrode pads are left out to serve as mound-like members;
D. a step of forming a common electrode film connected with the respective electrode pads through the respective openings on the entire surface of the semiconductor substrate provided with the insulation film formed thereon;
E. a step of applying a second photosensitive resin on the entire surface of the common electrode film;
F. a step of patterning the second photosensitive resin by use of photolithography treatment such that an opening substantially in the same size as the respective electrode pads is formed over the respective electrode pads;
G. a step of forming a bump on the common electrode film inside the respective openings of the second photosensitive resin by means of plating;
H. a step of removing the second photosensitive resin;
I. a step of forming a lower electrode on the respective electrode pads by patterning the common electrode film by means of etching thereof using the respective bumps as etching masks; and
J. a step of cutting the semiconductor substrate into semiconductor chips for individual semiconductor devices.
Further, the steps from A to C of the method of fabricating the semiconductor device may be replaced with the following steps comprising:
a step of preparing a semiconductor substrate provided with integrated circuits for a plurality of semiconductor chips and a plurality of electrode pads for connecting the integrated circuits with an external circuit, and forming an insulation film on the entire surface of the semiconductor substrate;
a step of applying a first photosensitive resin to the entire surface of the insulation film;
a step of patterning the first photosensitive resin by use of photolithography treatment such that an opening in a ring-like shape is formed over the respective electrode pads;
a step of forming an opening in a ring-like shape in portions of the insulation film over the respective electrode pads by patterning the insulation film by means of etching thereof using the first photosensitive resin as a mask; and
a step of removing the first photosensitive resin.
Furthermore, the steps B and C of the above-described method of fabricating the semiconductor device may be replaced with the following steps comprising:
a step of forming an electrically conductive film thicker than the insulation film on the entire surface of the semiconductor substrate provided with the insulation film formed thereon;
a step of applying a first photosensitive resin to the entire surface of the electrically conductive film;
a step of patterning the first photosensitive resin by use of photolithography treatment such that portions thereof only on top of the central part of the respective electrode pads are left out;
a step of patterning the electrically conductive film through etching thereof using the portions of the first photosensitive resin as masks such that portions of the electrically conductive film only on top of the central part of the respective electrode pads are left out to serve as mound-like members; and
a step of removing the first photosensitive resin.
Otherwise, the steps from B to D of the above-described method of fabricating the semiconductor device may be replaced with the following steps comprising:
a step of forming a common electrode film connected with the respective electrode pads through the respective openings on the entire face of the semiconductor substrate provided with the insulation film formed thereon;
a step of forming an electrically conductive film on the entire surface of the common electrode film to a thickness thicker than the insulation film;
a step of applying a first photosensitive resin to the entire surface of the electrically conductive film;
a step of patterning the first photosensitive resin by use of photolithography treatment such that only portions thereof on the electrically conductive film corresponding to the central part of the respective electrode pads are left out;
a step of patterning the electrically conductive film through etching thereof using the portions of the first photosensitive resin as masks such that only portions of the electrically conductive film on the common electrode film corresponding to the central part of the respective electrode pads are left out to serve as mound-like members; and
a step of removing the first photosensitive resin.
Still further, the steps B and C of the above-described method of fabricating the semiconductor device may be replaced with the following steps comprising:
a step of applying a first photosensitive resin to the entire surface of the semiconductor substrate with the insulation film formed thereon;
a step of patterning the first photosensitive resin by use of photolithography treatment such that an opening is formed only on top of the central part of the respective electrode pads;
a step of forming a mound-like member composed of a metal film by means of plating on the respective electrode pads inside the respective openings of the first photosensitive resin; and
a step of removing the first photosensitive resin.
With the respective methods of fabricating the semiconductor device described above, a step of lowering hardness of the respective bumps by applying heat thereto is preferably performed between the step of forming the lower electrodes and the step of cutting the semiconductor substrate.
In such a case, in the step of forming bumps, the respective bumps are preferably formed of gold, and in the step of lowering the hardness of the respective bumps, the application of heat is preferably performed at a temperature in a range of 250xc2x0 C. to 350xc2x0 C. in a furnace into which nitrogen gas is introduced so as to render Vickers hardness of the respective bumps to be in a range of 40 to 60.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.